Nucleic acids have evoked extensive interest as potential therapeutical agents, e.g. as antisense agents, aptamers, ribozymes and duplexes for gene therapy. There are a wide range of specific application of such agents which are made from RNA, DNA or modified nucleotides. One specific goal is to develop those DNA aptamers which target peptides that have significant roles in cancer or thrombosis and which allow non-invasive imaging with Positron Emission Tomography (PET). Several crucial questions regarding stability, and tissues or cellular distribution must be fully addressed, which makes the tritiated aptamer desirable. 2'-bromo-d-ATP (10 mg) was tritiated using T2 and PdO in phosphate buffer (pH 8.6) to yield 200 mCi of 2-3H-dATP of specific activity 26 Ci/mmole. This tritiated dATP was used to produce a 16-mer thrombin aptamer using a 33 base template in the presence of DNA polymerase. After desalting and base hydrolysis 670 _Ci of 3H aptamer was isolated. Gel electrophoresis and 3H NMR verified the purity of the tritiated 16-mer aptamer. Binding studies have indicated that the affinity of 3H-16-mer analog for thrombin is nearly the same as measured for the original aptamer. Stability of the 3H-aptamer in blood has been examined in isolated rabbit blood utilizing phenol/chloroform extractions and ion exchange HPLC. The aptamer was degraded in the blood, with a biological half-life of between 15-30 minutes. The HPLC retention patterns indicate that degradation occurs at both the 3'- and 5'-termini. Though this degradative process is relatively rapid, and would require a short-lived PET isotope for clinical use (e.g. 11C), the rapid blood clearance could enhance imaging of thrombi in blood vessels. It is also conceivable that aptamer bound to thrombin will be more stable in vivo. This possibility is currently being studied with a deendothelialized aorta rabbit model, the 3H-aptamer, and chromatographic and autoradiographic methods to determine both the specificity of the aptamer to thrombin and the stability of the aptamer bound to thrombin in blood clots.